This invention is suited for use in ink-jet printers in which a print head scans over a print medium, such as a sheet of paper or transparent film, by shuttling bidirectionally across the print medium or by moving continuously along the print medium in one direction while the print medium is supported against a rotating drum. Printed images are formed by selectively depositing ink drops of primary or base colors at uniformly spaced address locations disposed in uniformly spaced rows to form a dot-matrix image. Variations in color may be achieved by depositing ink drops at the address locations by using well-known dithering or gray-scale techniques.
This invention is equally applicable to any printing process in which a print head travels along parallel lines relative to a print medium to form a desired image, whether the image is primarily graphic or textual. The term "printing" includes a general situation in which a print element or nozzle addresses an ink drop location, without regard to whether ink is actually deposited. Moreover, in the general situation the size of the drop may vary and even the number of drops of a given color that are deposited at a particular address location may vary.
Skilled workers recognize that printing speed may be improved by printing more than one line at a time by ejecting ink drops from multiple nozzles that are configured in a linear array such that a band of lines are printed during each scan. Such printing is referred to as band printing.
In color band printing, it is desirable that ink-jet arrays for ejecting different colors be spaced apart in the direction of print medium movement so that each color dries or sets before the next color is deposited. With this configuration, multiple spaced apart bands of colors are deposited in the same sequence for both directions of print head scanning relative to the print medium. However, print heads having such an array configuration have a relatively large dimension in the direction of paper movement, thereby limiting their usefulness to printing on relatively flat print media. Such a configuration can also limit how close to an edge of a print medium printing can be achieved.
Because it is common to support print media on a drum, ink-jet arrays are commonly spaced apart in the direction of scanning to reduce the print head dimension in the direction of media movement. In this case, multiple bands of colors are deposited one on top of the another during each scan of the print head, with an ink color laydown sequence being dependent on the direction of scanning.
Both configurations have advantages and disadvantages that are related to a variety of printing variables as described in more detail below.
Prints generated by some color ink-jet printers exhibit noticeable streaks parallel to the print head scanning direction in areas printed with solid color fill. The streaks can be either higher or lower in optical density than the surrounding area, and they occur where a band of color printed during one scan abuts a band of color printed during a subsequent scan. Streaks may be caused by mechanical positioning errors in paper-advance mechanisms or ink bleeding between bands. To minimize streaks, the bands of color should be interlaced rather than abutted.
Color band interlacing refers to the partial overlapping of a first printed band of a color with a subsequent printed band of the same color. This also requires line interlacing and results in the spacing apart of any printing defects due, for example, to a defective ink-jet in an array of ink-jets.
Line interlacing entails printing adjacent lines of dots of a particular color during sequential scans of the print head. For example, lines 1, 3, 5, etc., are printed during a first scan, and lines 2, 4, 6, etc., are printed during the next scan. In a high-speed printer, it is desirable to print during both scanning directions. With line interlacing, any printing errors and related image defects that are dependent on the scanning direction are generated at a spatial frequency that is the inverse of the spacing between lines.
Streaks and banding effects can also be caused by the type of ink ejected by a print head, such as water-based inks, oil-based inks, and phase-change or thermoplastic inks. Phase-change inks are preferred, because of their color intensity, "drying" characteristics, and compatibility with many types of print media including plain paper.
Phase-change inks, are typically supplied to a printer in solid forms such as sticks or granules, are melted by a heater, and ejected toward the print medium by the print head as hot liquid ink droplets. When the hot ink droplets strike the print medium they cool, changing state back to a solid form (setting), and bonding to the print medium in the process.
U.S. Pat. No. 5,075,689 issued Dec. 24, 1991 for BIDIRECTIONAL HOT MELT INK JET PRINTING describes a phase-change ink-jet printer in which printed color hue is dependent on the order in which inks are deposited one on top of the other. If a first colored ink drop is deposited and a second colored ink drop is deposited on top of the first drop, a particular color is created. But if the ink color laydown sequence is reversed, a slightly different color is created. The patent proposes depositing both drops in such a short time period that they remain in a liquid state that allows their colors to mix together prior to setting. However, this solution is not satisfactory for all phase-change inks, especially those having high chromaticity. Moreover, because pairs of liquid drops that mix together form a larger resultant drop than that in which the second drop is deposited on top of a set drop, color hue shift effects are still noticeable.
Therefore, it is known that the ink color laydown sequence is important and, as described above, depends on scanning direction in some print head array configurations, ink composition, and time between depositing successive drops.
Ideally, to reduce hue-related printing artifacts, ink laydown sequences should always be the same regardless of scan direction. If this is not possible, an alternative is to alternate the ink laydown sequences on adjacent lines so that the hue variations will have a high spatial frequency that is not easily perceived by the human eye.
It is desirable, therefore, to provide line and band interlacing of each of the colors and a constant color laydown sequence when printing bidirectionally. As described above, a dimensional limitation is often imposed on the height of ink-jet nozzle array configurations. There are also print head manufacturing limitations to the closeness of nozzle and array spacing. Skilled workers might conclude that an ideal print head would have nozzles and arrays spaced closely together and provide the desired print interlacing. Another worker might require the arrays to be widely separated in the scanning direction to allow a first drop to set before a subsequent drop of a different color is deposited over the first drop.
Because of the wide variety of nozzle array configurations, ink types, print media supports, print head and media movement mechanisms, and the like, a corresponding variety of print interlacing methods and print head nozzle array patterns are known in the art.
For example, U.S. Pat. No. 5,070,345 issued Dec. 3, 1991 for INTERLACED INK JET PRINTING characterizes many of the banding and seaming problems associated with phase-change ink-jet printing and describes guidelines for minimizing those problems. The guidelines state that banding can be minimized if adjacent dot rows are not printed during the same pass, and each dot row should be deposited between either unprinted adjacent dot rows or deposited between adjacent printed dot rows. Thereby, printing artifacts caused by ink blending and thermal unbalance problems are minimized. Nozzle array configurations and printing methods are described with reference to FIGS. 1-4 that conform to the guidelines.
FIG. 1 shows a first nozzle array configuration 10 that is split into two 8-nozzle subsections 12 and 14. Nozzles 16 in each subsection are spaced apart vertically by two line widths 2 V, and subsections 12 and 14 are spaced apart vertically by three line widths 3 V.
FIGS. 2A-2B show a printing method suitable for use with first nozzle array configuration 10. An even number, in this case 16, of nozzles spans 32 lines. The printing method proceeds as follows:
During a first pass in a first direction, nozzles 1-18 are disabled and nozzles 9-16 are enabled for printing even-numbered lines 18-32. Enabled nozzles are shown as darkened circles, and disabled nozzles are shown as open circles.
Array 10 is stepped down 16 lines relative to the print medium.
During a second pass in a second direction, nozzles 1-8 are enabled for printing odd-numbered lines 17-31 and nozzles 9-16 are enabled for printing even-numbered lines 34-48.
Array 10 is stepped down another 16 lines relative to the print medium.
During a third pass in the first direction, nozzles 1-8 are enabled for printing odd-numbered lines 33-47 and nozzles 9-16 are enabled for printing even-numbered lines 50-64.
Array 10 is stepped down another 16 lines relative to the print medium, and the process is repeated as required.
Advantages associated with array 10 and its printing method include uniform 16--16--16 line print head stepping, full nozzle utilization, and uniform interlacing. Disadvantages include print head manufacturing difficulties and print head positioning restrictions related to array subsection spacing 3 V.
To overcome the above-described disadvantages, FIG. 3 shows a second nozzle array configuration 20 in which all 16 of nozzles 16 are spaced apart vertically by two line widths 2 V to form a linear array. Note that in nozzle array configurations 10 and 20, nozzles 16 are spaced apart horizontally by a distance H that is typically an integer multiple of the dot spacing in a scan line. Distance H is usually made as small as possible to facilitate print head manufacturability while still maintaining vertical spacing 2 V between nozzles 16.
FIGS. 4A-4B show a printing method suitable for use with second nozzle array configuration 20. Again, an even number of nozzles 16 is employed. However, for nozzle array configuration 20, nozzles 16 span 31 lines. The printing method proceeds as follows:
During a first pass in a first direction, nozzle 1 is disabled and nozzles 2-16 are enabled for printing odd-numbered lines 3-31.
Array 20 is stepped down one line relative to the print medium.
During a second pass in a second direction, nozzle 16 is disabled and nozzles 1-15 are enabled for printing even-numbered lines 2-30.
Array 20 is stepped down 29 lines relative to the print medium.
During a third pass in the first direction, nozzle 1 is disabled and nozzles 2-16 are enabled for printing odd-numbered lines 33-61.
Array 10 is stepped down another one line relative to the print medium, and the process is repeated as required.
Advantages associated with array 20 and its printing method include print head manufacturability and no stepping restrictions. Disadvantages include nonuniform 1-29--1-29 line print head stepping, incomplete nozzle utilization, nonuniform line interlacing, and no band interlacing. The uneven print head stepping can cause uneven mechanical positioning and thermal imbalances that cause banding.
Color ink-jet printing is discussed in U.S. Pat. No. 5,079,571 issued Jan. 7, 1992 for INTERLACED PRINTING USING SPACED PRINT ARRAYS, assigned to the assignee of this application, which describes the utilization of uniform linear arrays, each having an even number of nozzles. Each array is configured for color interleaving such that no two colors are printed on the same line during the same scan. Ink laydown order and color blending problems are thereby minimized. However, the print head internal architecture is complex and nonuniform, leading to ink purging, crosstalk, manufacturability, and banding problems. Moreover, performance is limited because the arrays are spread vertically, placing a limit on the number of nozzles in each array.
Despite many prior attempts, banding, seaming, and streaking problems persist in color ink-jet printing. Moreover, the problems seem more pronounced in high-performance, readily manufacturable print heads having arrays with large numbers of nozzles.
What is needed, therefore, are color ink-jet printing methods and nozzle array configurations that minimize color printing artifacts when used with high-performance print heads that have multiple nozzle arrays, each having a large number of nozzles.